This invention relates to measurement or monitoring functions and in particular to measurement or monitoring functions associated with large liquid-containing vessels such as oil storage tanks.
One vital measurement or monitoring function is that of base plate deformation in such vessels, which deformation arises as follows.
One presently accepted common method of building large vessels for containing liquid, for example, storage tanks for the bulk storage of oil, uses a construction technique in which the foundations for the tank consist of a "ring" of compacted rock or concrete. Such a tank is known as a "soil supported" tank.
Unfortunately, soil supported tanks suffer from differential settlement of a number of kinds (axial, peripheral and base plate), and whilst the tank design can allow for some such settlement, it cannot cope with excessive amounts. Accordingly, in order to monitor the actual settlement, various measurements have to be taken of the tank's dimensions over a test period (commonly of at least a year) during which it is slowly filled, meter by meter, with liquid (commonly water) not only to check that the tank does not leak but also to assist in the satisfactory consolidation of the foundation sub-soil. Indeed such measurements have to be checked throughout the period that the tank is in service and the life of a typical bulk oil storage tank may well be in excess of twenty years. Measurement of axial and peripheral settlement can relatively easily be effected from outside the tank, but measurement of base plate settlement cannot, and presently causes much difficulty (part of which is that current methods are too simple to give good; detailed, results).